I'd have thought that AD2 redemtion should be valid all the time Cake bundle it with Sonar. You redeem against Sonar, this is the first Cake know about it, generates an AD code, you redeem. XLN have no purchase time other than when cake provided the registration code for XLN.
Even if you have no interest in AD2 at the moment, something may change with them, and therefore you. I would register yourself with XLN, and just not choose a product. I have 2 kit pieces available for download, as I couldn't decide at the time, they are still available for me from November.
I have the drums that I wanted, but since deciding to by Platinum I have rattled my brain trying to decided with new 3 too add, and there are 4 kits that interest me, but I am still uncertain. I think I will just sit on the option to download, and register as soon as I redeem against Cakewalk. There are apparently more kits planned for the year, so have some in the bank could make for a pleasant purchase when something comes along.